Simple disposable assay devices, based for example on the technology described in EP-A-291194, are used extensively today in a wide variety of analytical circumstances. These include assays used in the home to detect pregnancy or fertility, and assays used in clinics and doctors offices to detect a wide range of diseases and physiological conditions. Typically these assay devices comprise a dry strip of porous material containing various reagents. The strip can be a single length of material, or can be a composite of various materials arranged in series. A sample liquid is applied to one end of the strip and while it permeates towards the other end, picks up one or more reagents and carries them into a detection zone further along the strip. There may also be a control zone, usually located downstream from the detection zone. Usually the assay result signal and the control signal are created by the accumulation of labelled material in the respective zones. Typically this labelled material comprises a particulate direct label, such as a gold sol or coloured latex micro-particle. Usually the assay strip is protected within a casing, constructed for example from one or more plastics mouldings. However, in some products no casing is provided.
To satisfy the demand for such devices, the test strips must be manufactured in very large numbers. This is achieved by applying to a large sheet or long length of the porous material the various reagents in appropriate locations so that sub-division of the sheet or length into individual identical test strips can be accomplished at a later stage of the manufacturing process. Typically the zone or zones are created by applying appropriate reagents onto the porous carrier by remotely controlled pens or ink-jet printers, which deposit the reagents as lines or as a series of dots. Depending on the reagents used and the nature of the porous carrier material, a chemical pre-treatment procedure may be required. Usually following reagent deposition and drying or fixing, the porous material will need to be "blocked". Mobilisable reagents, such as a labelled reagent deposited elsewhere on the strip, can be applied using similar equipment. A typical manufacturing procedure will therefore entail the bulk portion of porous carrier material being passed through various reagent depositing stations with intermediate stations providing other treatments (eg. blocking) as required. After final drying or other treatment, the porous material is cut or otherwise sub-divided into the individual strips. These procedures can utilise either single large sheets of the porous carrier material or a "continuous" roll. Various lamination or backing materials can also be applied to the porous carrier material while this strip-forming sequence of operations is conducted.
Because such assay strips are required in ever-increasing numbers, there is a need to streamline these operating procedures both to increase throughput and to minimise differences between product batches.
According to one established manufacturing process, in which separate sheets of porous carrier are used, some of the treatments such as blocking are achieved by immersing the whole sheet at an appropriate stage in a bath of blocking solution, followed by draining and drying before further processing. This has the disadvantage that trace amounts of materials can be leached from each sheet upon immersion in the blocking bath. For example, previously deposited reagents may not be completely fixed to the carrier. Moreover, as the blocking agent is progressively consumed during each immersion, the amount of blocking agent applied is not constant. The composition of the blocking solution can therefore change over time, and indeed cause slight contamination of the sheets as leached materials accumulate which may result in a background of non-specific binding when the eventual assay is conducted. Another disadvantage of immersion is that this tends to be a "quick dip", in the interest of manufacturing throughput. However, reagents already deposited in the detection and/or control zones can affect the wetting properties of the porous material at that critical location, and impair the blocking process there, which may lead to variable quality of the signal generated during an eventual assay. A slow blocking procedure, intended to overcome this, introduces delay in the manufacturing process. Similar immersion may form part of a process using a "continuous" long length of carrier material, with comparable disadvantages.
It is desirable to ensure that previously deposited reagents on the carrier material are not dislodged by later processing steps and relocated in regions of the strip wherein non-specific binding could be disadvantageous during use.
It is therefore an object of the invention to provide a manufacturing process for assay strips which especially enables blocking of the strip material to be accomplished more readily and without significant potential disruption to the final assay performance.